Abstract
Bioclimatic and physiological differences between sun and shade needles suggest that their chemical composition should differ. An analysis of resin acid content demonstrated that although the same resin acids were found (i.e., levopimaric/palustric, dehydroabietic, abietic, neoabietic, 4-epiimbricatolic, pinifolic, and isopimaric acids), both individual and total resin concentrations were significantly higher in sun than in shade needles. These relationships were true for a dense 40-year-old pine stand in southern Sweden as well as for a dense 12-year-old stand in northern Sweden. The role of resin acids in defence against needle-eating insects and the relative susceptibilities of shade and sun needles to needle-attacking organisms are discussed. Variation in resin acid concentration in sun and shade needles should be considered when such acids are used as genetic markers.
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